THE INTRINSIC ABILITY OF RIBOSOMES TO BIND TO ENDOPLASMIC-RETICULUM MEMBRANES IS REGULATED BY SIGNAL RECOGNITION PARTICLE AND NASCENT-POLYPEYTIDE-ASSOCIATED COMPLEX

Signal peptides direct the cotranslational targeting of nascent polype ptides to the endoplasmic reticulum (ER). It is currently believed tha t the signal recognition particle (SRP) mediates this targeting by fir st binding to signal peptides and then by directing the ribosome/nasce nt chain/SRP complex to the SRP receptor at the ER. We show that ribos omes can mediate targeting by directly binding to translocation sites, When purified away from cytosolic factors, including SRP and nascent- polypeptide-associated complex (NAG), in vitro assembled translation i ntermediates representing ribosome/nascent-chain complexes efficiently bound to microsomal membranes, and their nascent polypeptides could s ubsequently be efficiently translocated, Because removal of cytosolic factors from the ribbsome/nascent-chain complexes also resulted tn mis targeting of signalless nascent polypeptides, we previously investigat ed whether readdition of cytosolic factors, such as NAC and SRP, could restore fidelity to targeting. Without SRP, NAC prevented all nascent -chain-containing ribosomes from binding to the ER membrane. Furthermo re, SRP prevented NAC from blocking ribosome-membrane association only when the nascent polypeptide contained a signal. Thus, NAC is a globa l ribosome-binding prevention factor regulated in activity by signal-p eptide-directed SRP binding, A model presents ribosomes as the targeti ng vectors for delivering nascent polypeptides to translocation sites. In conjunction with signal peptides, SRP and NAC contribute to this s pecificity of ribosomal function by regulating exposure of a ribosomal membrane attachment site that binds to receptors in the ER membrane.